Synchronizing separator for television receivers



Nov. 4, 1952 E. COHEN ySYNCHRONIIZING SEPARATOR AFOR TELEVISIONRECEIVERS -Fled May 19, 1949 Patented Nov. 4, 1952 SYNCHRONIZINGSEPARATOR FOR TELEVISION RECEIVERS Elias Cohen, `Philadelphia, Pa.,assignor to Philco Corporation, Philadelphia, Pa., a corporation ofPennsylvania Application May 19, 1949, Serial N0.`94,18 7

The invention hereindescribed and claimed relates to asynchronizing-signal separator circuit fora televisionreceiver. Y In atelevisionV receiver, it is the function of the synchronizing-signalseparator circuit to derive," usually from the detected video signal,the

" horizontal `and Vvertical synchronizing pulses which extend from theblanking signal into the blacker-than-black region. The separatedsynchronizing pulses are then applied to, and control the action-of,sweep circuits which effect the horizontal and vertical deflections ofthecathoderay-tube beam.

A large variety of synchronizing-signal separator circuits have beenprovided by the prior art. Some of the prior-art separator circuits haveexcellent operating characteristics but have t the v`disadvantage ofbeing complex and expensive. Others, which are more simple in struc--ture and less expensive, have not been entirely satisfactory insofar as-their 'operating characteristicsare concerned. The present inventionprovides an improved separator circuit of the simple, relativelyinexpensive, type. f

Some of the prior-art separator circuits ern- 1 ployinput networkshaving relatively long time constants, as for example, of the order of.onetenthj of a second. Others employ input net- `works havingsubstantially shorter ltime constants, as for example, of the order ofone-thousandth of a second. The present invention providesaniimprovedseparator circuit of the short- Atune-constant type.A`5,..Theshort-timeeconstant typeof separator circuit is ordinarily`preferable when the sweep circuits ofthe-television receiver includeanv oscil- Alatoru-'Which is stabilized by the average repetitionfrequency ofthe. synchronizing pulses. LPrior-'art-separator circuits ofthe short-timeconstant-ftype have not, however, been entirely*lsatisfactory. It has been noticed that their operationis disturbed to asubstantially greater extent byshort bursts of high-amplitude noise thanl is `theoperation of thelong-time-constant type hof separator circuit.The reason for the greater stability of the'long-time-constant separatorcircuit in the presence of high-amplitude noise of short duration is!that the large series input capacitorv of the*long-time-constantycircuit is charged to buta fraction of the peak voltage of th appliedlnoise pulse. Nevertheless, the short- 'time-constant separator circuitis preferable in atleast certain-applications, as has been indicatedabove.

" I 'The' lproblem #involved is most serious uwhen A1 claim. (c1. 17a-7.3)

` the television receiver is located in an electrically noisy area.

' arator circuit is so disturbed that intermittent but persistenttear-out of horizontal lines occur, I together with an occasional shiftof the picture frame vertically. l In other locations, where the I noisepulses are of lesseramplitude and occur less frequently, the problem isless serious. Nevertheless, even the occasional tear-out of a relativelysmall number of picture lines is objectionable.

' tioning in an improved mannerunder conditions -of low signal strengthand high-amplitude noise.

The foregoingobjectsare attained by modifying the prior-artseparator'circuit to return the grid leak, not to cathode eitherdirectlyor by way of ground as is conventional, `but to a source ofpotential substantially more positive than the cathode potential.v

The objects, featuresand advantagesof the invention, and the mannery inwhich the objects are attained, will be most. readily understood byconsidering the followingdetailed description and accompanyingsinglefigure. of drawing wherein isfillustrated in partly diagrammatic,partly schematic, forman otherwise conventional vtelevision receiverinto which the improvedsynchronizing-signal separator circuit of thepresent invention has been incorporated.

Referring nowv to the drawing, there is shown a television receiverwhich includes an antenna I. I0, an R-F amplifier stage II, a frequencyconverter stage I2, and a first stage of audio and video I-Famplification I3. AFollowing the audio and video I-F amplifier stage I3,the audio and vdeo AI-F signals are separated into different channels.The audio channel may comprise an audio I-F amplifier I4, an audiodetector I5, an audio amplifier I 6, and a loudspeaker Il, While thevideo channel may comprise several additional stages of video I-Famplifier I8, a video detector I9, a video amplifier 20 and a picturetube 2l. J y 2 A portion of the output of the video Vamplifier 20 is fedto a synchronizing-pulse amplifier 22 whose functions includef-thatofinvertingV the There, the normal action of the seppolarity of the videosignal from one in which the synchronizing pulses extend negatively toone in which the said pulses extend positively, as is indicateddiagrammatically in the drawing. The output of the synchronizing-pulseamplier 22 is applied to the synchronizing-pulse separator` circuitshown `schematically in the v"drawing VAand suitably marked. .Thesynchronizing pulses derived by the separator circuit are supplied, byWay of an inverter circuit 23, to the sweep circuits 24, and thesaw-tooth signals developedby the sweep circuits 24 are applied'. :to.picture tube 2|.

Except for the synchronizing-signal:separator circuit, into which theimprovementoi triep-present invention has been incorporated, both thestructure and operation of thetelevisionreceiver shown in the drawing isentirely conventional and will not be described further.

A portion of the synchronizing-signal sepiaratorY circuit isalso.conventional and :will bede- Hscribed iflrst. i The. separatortube'maybeeither atriode,` pentode,` or .other type of tube. `Inthe drawing aytriode. is shown Whose .plate is connected to a: source of .positivesupply voltage, B+, by Way of a voltage-divider circuit. Thevoltage-.divider circuit comprises resistors 32 and 33 of which resistor32v is ofsubstantially larger magnitude. In a typical.case,. the valueof resistor'32 maybe 60,000.0hmsand resistor 33 may -be 10,000 ohms.Then,iif.the1full B-lsupply voltagelbe 350 volts, the. Dy-C.plate-voltage on Y the-separator tube `SEI .Willfbefty volts. theorderof magnitude offvoltageconventionally This is employed on the plate ofa1.synchronizingsig vrial separator tube, a lowplate voltage :beingemployed so.that.lthe,applied synchronizing pulses,

;.even ofj.relatively weak signals, may .be able to :drive the; tube:from cutoffpto' .plate-.currentfsat- .uration. '.The -clipping action.effected by plate- .current saturation `.providesoutput' pulses of uni-.fo1m;amplitude. from .applied Apulses Whose am--p'litudes',mayalaryrelatively widely.

i'lhe voltage-.divider resistorsuSZ and .33 also .function as 'the:loadsnetwork :for .the plate cir- .cuit-.fof the :separator tube.Thezcathode of the separator tube is'connecteddirectly :to ground.

The video signal, whose synchronizing-'pulses extend in. a. positivedirection, is applied; tothe .gridzof the sync-separator :tube 'by fway..of a

..couplingfcapacitor 34.

.That .portion of the 'fsynchronizingesignallsep- Inaccordance with byinvention, however,V the; grid leak is not returned to-cathode,either.directly or by Y Way-of ground, but' is returned .to a 'source ofD.-C. voltage of suitable magnitude substantiallymcre positive thanthecathode potential. It Will frequently happen that the-D.-C. voltageapplied to theplate of the sync-'separator tube 'w'ill 'be Otsuitablemagnitude,4 and, since this voltage isreadilyavailable, theirgridleak-=35,. in

the v preferred embodiment .of rrny...invention, is

'returnedito the :plate of"v the sync-separator tube.

- However.. the :grid .leak may, in 'other-instances, be returned .toother sources -of positive voltage@ cof suitablefmagnitudeyeitherspecially provided or available. An example of the latter is'the screengrid voltage, :Where a pentode isemployed LintheiseparatoncircuitTheimprovementitdbe derived: iromtheabove-r described departure fromconventional practice is substantial and important, as will becomeclear.

It was stated previously hereinabove that my invention relates to animprovement in a shorttime-constant type of sync-separator circuit:which ordinarily employsatimeconstant of the order ofone-thousandth ofa second.

In the circuit illustrated in the drawing, capacitor is shown to have avalue of 330 micromicrofarads and resistor 35 a value of 4.7 .megohms Insuch case, the time constant of the combination would be 1551microseconds, which isequivalentto about 24 line periods. Theivaluesishown.areintended to be typical, and not limiting.

...Insofaras the time constant is concerne-d, the improved circuit ofthe present invention does not...difer from the prior-artshort-time-constant separator circuit. In both cases, the time constantof the :capacitor-resistor,networkis requiredttobe long-.relativetoaline period,wbut

- .cuitfbothof the prionart and of the presentin- Vvention, the RC inputnetwork exerts aihigh-pass ltering action so that, in the absencewof-theseparator tube; .the vertical. synchronizing pulses would not pass.through. However, the-.grid- 'fcath'ode portionof the. separator tube,actingas a diode and in .cooperation with. the.co.upling.ca :pacitor andgrid:,leak, effects restorationfof the f D.C. .andlow-frequency#components of .the video signal.n Consequently, the.Lvertical synchronizing pulses areipresentinzthe output of thelseparator circuit.

Before. discussing .f the advantages of vthe improved circuit, it will.be helpful to-.considerthe roperation -of theprior-art..short-time-constant separator circuit. In'theprior-artcircuit, the grid leak isconnected-.to cathode, ordinarily .byway of ground. The video signal applied to-the separator circuit is ofsuch polarity that the synchronizing pulses extend inthe positivedirection. In .the absence of an applied fvideosignal, theseparator tubeof. theprior artziszero biased. The applied video signal drives.itheggrid-.positive,v grid current, flows;y .and the couplingl'capacitor is .rapidly charged.negatively .to a; .magnitudeV`corresponding-'to the-.positive peak` amplitude Lof the appliedAvideoysignal ias measured from. the average signal level, i. e., .thecapacitor chargeszto a Voltage corresponding'to thetips of .thevsynchronizing pulses. Thelshunt `grid 'leak is of high .resistance and"thefcharge-ileaks off very .slowly during the intervalsabetweensynchronizing pulses.. Consequently, theinput': network of theseparatortube levels onrthe tips of .the synchronizing .pulses andthe.bias voltage 'developed acrossthe gridleak prevents platecuri rentfromilowing .except duringthel brief synchronizing-pulse intervals. Thetime constant of the.v `capacitor-resistor 'input circuit `beinglongcompared tol a` line period. vthere vis no substantial varation :in thegrid-.bias from `line to line. and jin the absence; ,of .strongnoisefpulsesfthe prior-art separator `circuit yoperates in- 1asatisfactory manner. u

, Considermovkwhat -fhapnensrinfthenprior-art circuit, when strong noisepulses are present. The signal applied'to the grid ofthe separator tubethen includes positive noise. pulses whose amplitudes may be severaltimes the amplitude of the video signal. Moreover, al number of suchnoise pulses may occur in a single line period. Under theseeonditions,the right-hand plate of" the coupling capacitor is charged negatively tosubstantially the peak value of the noise impulse asv measured from theaverage signal level, the input impedancevof the tube being very low inthe positive-grid region. The negative charge thus developed ,across thecoupling capacitor acts to bias the separator tube far belowplate-current cutoff. And, since the RC time constant of the couplingcapacitor andthe grid leak may be of the order of twenty line Vperiods,a substantial number of line periods may pass before the capacitor isdischarged suiciently to permit the separator circuit to function in anormal manner.

As indicated hereinbefore, the present invention proposes that ther-gridleak be returned to a source of voltage of selected magnitudesubstantially more positive than the cathode potential. Preferably, thegrid leak may be returned to the plate of the sync-separator tube. Whenthis is done, the coupling capacitor 34 discharges during the intervalbetween synchronizing pulses, not toward cathode or ground potential asin the prior-art circuits, but toward the voltage of selected magnitudesubstantially more positive than the cathode. Then, when a noise burstoccurs and coupling capacitor 34 is charged far negative, the timerequired for the capacitor to discharge to its normal operating value issubstantially shorter than where the capacitor discharges toward cathodeor g-round potential. As

a consequence, the number of synchronizing` pulses which are lostfollowing a noise burst is substantially reduced.

The improvements accomplished by my invention are particularly greatwhere the applied video signal is of low signal strength. In the priorart circuit, the difference in time required for the separator circuitto resume normal operation, following strong electrical noisedisturbance, is a function of video signal strength, being longer wherethe video signal strength is low than where it is high. This is due tothe fact that the negative bias developed across the grid leak is adirect function of video signal strength. Consequently, when thecoupling capacitor is charged far negative as a result of a burst ofelectrical noi-se, the capacitor must discharge to an operating biaswhich, in the case of a video signal of low strength, is relativelyclose to ground potential. And, as is well known, the time required fora capacitor which is being discharged to ground to reach a value closetoground potential is very long.

In my improved circuit, the time required for the separator circuit tofunction normally, following a strong electrical noise disturbance, isnot substantially' different for low-strength video signals than forstrong signals. This is due to the fact that the capacitor isdischarging to a positive voltage whose value is relatively far removedfrom that of the negativebias which corresponds to normal operation ofthe separator circuit. And this is so, even though the biascorresponding to normal operation be relatively close to groundpotential, as in the case of video signals of low signal strength.

Stated graphically, in my improved circuit, foleration is reestablishedis located on a portion "of the discharge curve which is relativelysteep in comparison with that of the prior-#art circuit. As aconsequence, the bias corresponding to nor- "mal separator-circuitoperation is reached relatively quickly by the discharging capacitor.

l.I have found that the circuit ofthe present vinvention effects anadditional improvement entirely apart from that eiected 'duringhigh-amplitude noise disturbances. I have found that,

for low-strength video'signals, thernew circuit improves verysubstantially the'low-irequency restoring action of the separatorcircuit.

In the prior-art circuit, during the occurrence of thevertical-synchronizing-pulse portion of a low-strength video signal, thecoupling capacitor discharges toward ground from a negative bias whichis relatively close to ground potential. Consequently, the dischargetakes place along that portion of the discharge curve which isrelatively flat and the restoring action is not adequate to permit platecurrent to flow except perhaps on the very tips of thevertical-synchronizing pulses. superimposed upon the tips of thevertical-synchronizing pulses is considerable random noise of smallamplitude due, for example, to thermal agitation unavoidably present inthe circuit. As a consequence the vertical-synchronizing pulsesdeveloped in the output circuit of the separator do not containsufficient energy to insure positive control of the sweep-oscillatorcircuits.

The above situation is remedied in my improved circuit by reason of thefact that, during the occurrence of the vertical-synchronizing pulses,the capacitor discharges, not toward ground potential but toward theselected positive voltage. The discharge, therefore, takes place alongthe relatively steep portion of the -discharge curve and the restoringaction oi the separator circuit is substantially improved.

With respect now to the magnitude of the positive voltage to which, inaccordance with my invention, the grid leak is returned, the voltageshould be sufciently large to accomplish a substantial increase in therate at which the coupling capacitor is discharged, particularly whenthe video signal strength is low. On the other hand, the voltage shouldnot be so large that, in the absence of noise impulses, the couplingcapacitor discharges so rapidly in the intervals between synchronizingpulses that plate current flows during the blanking signal. The optimumvalue of positive voltage for a particular situation may be readilyascertained by experimentation. I have found, in general, that if thegrid leak 35 be returned to a positive D.C. voltage of the same order ofmagnitude as the negative bias developed on the grid of the separatortube by an applied video signal of minimum expected signal strength, avery noticeable and satisfactory improvement is accomplished withrespect to the time required for the separator circuit to recover fromelectrical noise disturbances.

Having described my invention, I claim:

In a television receiver having a source of videosignal wave, said waveincluding picture signals and synchronizing signals, said synchronizingsignals being of positive polarity; a separator circuit for derivingsynchronizing signals from said wave, said separator circuit comprising:a tube having at least cathode, grid and plate electrodes; a source ofpositive D.C. voltage connected to 7 ,-8 the' -plate of saidltube;a'-capaoitor zzoupling said REFERENCES VvCITED Sour'of' Vide'sgnalWave'tothe grd'of Sad The following references are rof record in thetube; a grid leak-connectedbetween the grid and me of; this patent:yplate of* said -tube;-and means, including :an im- ;peclanceintheplate-:cathode circuit ofsad tube, 5 UNITED STATES PATENTS kforderiving--an -output voltage, VSaid coupling Number Name Date.capaoitor,:-grid'leakland D.-C.vo1tage-source hav- 2,173,239 TolsonSept. 19, 1939 1 ing Ysuch yvalues that, bythe diode action ofsaid2,176,663 Browne et a1 Oct. 17, 1939 grid andl cathode electrodes incooperation with 2,210,523 Blumlein Aug. 6, 1940 saidcoupling capacitor'and grid leak, a negative 10 2,293,523 Bare et al. Aug- 18, 1942bias-is'developed on said grid effective to render FOREIGN PATENTS saidtube non-conductive during the intervals `that:pictureSignalsare appliedtheretmsaid tube Nugfm Greglggm F b Dzatelgs being adapted to bedriven/.in conduction by said L e synchronizingfsignals applied;`thereto. l5 OTHER REFERENCES A Ridervs TV-Manual, vol. #1, Motorola TV,pages ELIAS COHEN. 1-45, 16.`

